I shall shortly be taking delivery of a Dell 3020 desktop with Window 7 preinstalled. I'm wondering about the feasibility of keeping Windows on the disk because very occasionally I run across something (e.g. firmware update) that demands it.

In the past I've usually just installed OBSD and overwritten the Windows partition, but I'm wondering about the feasibility of keeping it (I understand Windows 7 can shrink its partition if necessary). It's a 500GB disk.

The OBSD faq implies that dual booting is a bit of a nightmare, and if that is the case I won't bother, but I thought it might be easier if Windows is already there. Any pointers or comments gratefully received.

Windows 7, 8 and 8.1 could install on a Legacy BIOS using MBR. However Windows 8, 8.1 and 10 were designed to install on the UEFI BIOS using the EFI bootloader and GPT."

OpenBSD use to provide sysutils/grub but it does not presently build.
If you have an MBR, the OpenBSD FAQ used to have an entry on adding an OpenBSD entry to the Windows Boot loader (likely archived somewhere). Otherwise, you will be moving/resizing partitions and using a 3rd party bootloader.

Personally I used Debian Gnu/Linux live-cd/pendrive to install Grub2 bootloader on MBR (UEFI was not supported by OpenBSD at that time). Grub2 lets me choose between Windows and full encrypted OpenBSD.

UEFI should be even simpler to dualboot, because UEFI by itself should have some form of detection and menu (IIRC you need to press some key on keyboard) to choose which OS you want to boot.

__________________
Signature: Furthermore, I consider that systemd must be destroyed.
Based on Latin oratorical phrase